1. Field of the Invention
This invention is in the field of recovery of zinc from a sulfidic zinc source by means of electrolysis and is primarily concerned with purifying the liquor obtained in the original leaching step to crystallize out harmful compounds in an economical way while producing residues which are not harmful.
2. Description of the Prior Art
This invention relates to a method for processing sulfidic zinc ores or concentrates by means of a primary leaching process utilizing acid from an electrolysis cell, followed by separation of liquor and primary leaching residue. The primary leaching residue is dried in a drying stage and is subsequently treated in a smelting furnace to produce slag and exhaust gases containing readily volatilizable metals and metal compounds including zinc oxide. The liquor is subjected to a further treatment for electrolysis for zinc recovery. The primary leaching is carried out at atmospheric pressure in the presence of Fe.sup.3+ ions, and the primary leaching residue is resmelted into a storable slag without lead and cadmium upon combustion of the elemental sulfur contained therein. A sulfidic residue occurs. Additional zinc values are obtained from the zinc components volatilized in the smelting operation. A process of this type has been described in German Patent Application No. P 30 31 007.7.
Zinc production methods heretofore known in the prior art have several disadvantages. For one, the raw material selection is limited because of the presence of undesirable elements. Secondly, the process normally requires a multi-stage leaching process which involves high expenditure for capital and operating costs. The residues produced in the leaching operation create considerable leaching, filtration, clarification and storage problems, particularly when valuable metals are to be largely recovered. The resulting leaching residues represent a waste product which is harmful to the environment. The harmful substances contained in the residue, consisting of compounds of arsenic, antimony thallium, and the like, are readily washed out by rain water and could lead to contamination of the ground water. This makes it necessary to store such materials at specially protected installations. For a zinc works producing 100,000 metric tons annually, the quantity of residue amounts to about 30,000 tons per year of hematite or about 40,000 tons per year of goethite or about 50,000 tons per year of jarosite. Storing these considerable quantities of residue provides increasing difficulties in the acquisition of storage areas.
When processing sulfidic zinc concentrates for electrolytic zinc production, the concentrates are generally roasted in order to convert the sulfide into the acid-soluble oxide form. The exhaust gas contains SO.sub.2 which for economic reasons should be processed into sulfuric acid. There must be a suitable market for the sulfuric acid product (about 2 tons of sulfuric acid are produced per ton of zinc) because storage facilities are limited and transport of the acid over long distances is not economical.
The roasted product in the conventional process is leached with cell acid from the electrolysis cell, iron residues are separated out, and the resulting neutral liquor is freed of elements such as copper, cobalt, nickel, aresenic and the like by means of liquor purification. The pure zinc sulfate solution proceeds to the electrolysis whereby the zinc is precipitated as metal. In order to obtain a high zinc extraction ratio, particularly from zinc ferrites formed in the roasting of the concentrates, the zinc must be placed in solution together with the iron. This is carried out by subjecting the liquor residues to a hot acid leaching after the original neutral leaching. Depending on the process variables, the dissolved iron is precipitated as jarosite, goethite or hematite and is storage in special storage areas.
It has also been suggested to carry out the removal of elemental sulfur from metal sulfides by means of pressure leaching in a sulfidic medium. Oxygen serves as an oxidizing agent for the sulfur formation. A process already introduced into industrial scale technology known as the Sherritt-Gordon process involves leaching zinc concentrates under pressure with oxygen at a pressure of about 10 bar and a temperature of 150.degree. C. A zinc sulfate solution is produced which is supplied to the electrolysis circulation and elemental sulfur is produced in liquid form. This method has an advantage over conventional roasting, leaching, and production of sulfuric acid only when there is no need for sulfuric acid. There are special disadvantages of the process which include the fact that the process must be carried out under pressure and that autoclaves are employed. The difficulty of separating elemental sulfur from the liquor residue containing valuable metals is also present. Direct extraction of sulfidic materials, however, has the advantage that the metals are brought directly into solution in the leaching process without previous roasting and thus conversion of the sulfur into the gaseous state, so that the sulfur can be elementally produced. This elemental sulfur can be stored for unlimited times and can be processed into sulfur-containing products depending on the market situation.
Methods for the pyrometallurgical processing of the liquor residues have also been proposed and partially employed on an industrial scale. These methods, however, exhibit disadvantages since the residues must be dried and pelletized for the further thermal processing. These process steps are highly expensive. Further, these processes require a high energy outlay in the form of metallurgical coke, oil or other fuels and cause the formation of dust and of exhaust gases containing SO.sub.2.
In our aforementioned previous German application, there is disclosed a direct extraction of sulfidic ores, concentrates or other metallurgical intermediate products as an alternative to conventional roasting, leaching, production of sulfuric acid and pyrometallurgical processing of leaching residues. The previous application discloses a method for processing sulfudic zinc ores wherein the liquor separated out after the primary leaching is subjected to a liquor purification before it is supplied to the electrolysis.
The formation of elemental sulfur without the application of pressure provides a sulfidic primary leaching of zinc concentrate upon the addition of Fe.sup.3+ ions according to the reaction: EQU ZnS+Fe.sub.2 (SO.sub.4).sub.3 =ZnSO.sub.4 +2FeSO.sub.4 +S.degree.
The conversion proceeds quickly and completely in the presence of large quantities of Fe.sup.3+ ions. An effective oxidation of Fe.sup.2+ into Fe.sup.3+ must occur in order to guarantee an adequate Fe.sup.3+ ion concentration. The oxidation proceeds with oxygen according to the equation: EQU 2FeSO.sub.4 +1/2O.sub.2 +H.sub.2 SO.sub.4 =Fe.sub.2 (SO.sub.4).sub.3 +H.sub.2 O
This conversion is greatly dependent on the oxygen pressure and on the Fe.sup.2+ ion concentration. Copper ions and activated carbon promote the reaction. In addition to elemental sulfur, the primary leaching residue contains valuable mirror metals and zinc.
The known prior art exhibits numerous problems which are still unresolved, namely:
(1) the zinc yield in the primary leaching is low; PA0 (2) a high Fe.sup.3+ ion concentration is required for the primary leaching; PA0 (3) the oxidation of Fe.sup.2+ ions into Fe.sup.3+ ions is very slow; PA0 (4) the separation of FeSO.sub.4 from the filtrate of the primary leaching step is difficult; PA0 (5) a suitable solution for the return of the Fe.sup.3+ into the leaching circulation has not yet been developed; PA0 (6) the storage problem according to the conventional method has not yet been resolved; PA0 (7) the separation of elemental sulfur from the primary leaching residue by means of flotation or hot filtration is very costly; PA0 (8) an optimum concept for processing the primary leaching residue for recovering valuable metals and the zinc content has not yet been developed. PA0 (a) a crystallization stage with the crystallization of the Fe.sup.2+ out of the liquor as a ferrous sulfate hydrate, FeSO.sub.4.xH.sub.2 O, PA0 (b) separation of the filtrate from the crystallization stage, PA0 (c) precipitating the remaining Fe.sup.2+ ions in a precipitation stage by means of neutralization and oxidation, PA0 (d) separation of the filtrate from the precipitation stage, and PA0 (e) a purification stage for the filtrate to bring it up to electrolysis purity.